It is well recognized in the art that such signals are subject to distortions due to the different laws of variation of attenuation and group delay with frequency. A measure of such distortion is given by the peak-to-average (PAR) ratio of a test pulse specially transmitted for this purpose, e.g., as described in an article entitle The PAR Meter: Application in Telecommunications Systems by John H. Fennick, IEEE Transactions on Communication Technology, Vol. Com-18, No. 1, February 1970, pp. 68 to 73. A simple way of determining the extent of such distortion is with the aid of a so-called eye pattern, e.g., as discussed in the book Data Transmission by Davey Bennett, McGraw-Hill Book Co., 1965, pp. 118 to 121, chapter 7-7. The eye pattern is a trace formed on an oscilloscope screen by samples periodically taken from an incoming data train, under the control of sync pulses extracted therefrom; a succession of pulses of the same polarity establishes the outer limits of the pattern whereas a changeover between positive and negative pulses results in distrotions determining the thickness of the trace. Thus, the height of the free area within that trace is indicative of the quality of transmission.